Loans and Allowance for Loan Losses - Recorded Investment Amounts of Troubled Debt Restructurings (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	$ 1	$ 28	$ 2	$ 28
Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	0	28	1	28
Consumer Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	1		1
Commercial and industrial | Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default		$ 28		$ 28
Commercial mortgage | Commercial Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	0		1
Residential mortgage and home equity | Consumer Loans Portfolio Segment [Member]
Troubled debt restructured loans
Total troubled debt restructured loans at date of default	$ 1		$ 1